1. Field of the Invention
The present invention generally relates to a color signal processing technique and a color profile creation technique applied to color image reproducing apparatuses, such as color facsimile machines, color printers, or color copiers. The present invention particularly relates to a technique required to convert input color image signals to chromatic signals suitable to color image output apparatuses.
2. Description of Related Art
In general, four color components, cyan (C), magenta (M), yellow (Y), and black (K), are used when reproducing color images in electrophotographic printing or inkjet recording. Black ink is used because using only three colors of yellow, magenta, and cyan cannot achieve satisfactory image reproduction, lacking sharp image contrast, due to insufficient coloration. On the other hand, input color signals are typically represented by three components of a three-dimensional color space, such as the L*a*b* color space or the L*u*v* color space independent of devices and equipment, or the RGB color space used for monitoring signals. For this reason, the color signals have to be converted from the three-dimensional color space to the four-dimensional color space when reproducing a color image. However, since there are a number of CMYK signal levels equivalent to a three-dimensional input color signal, a problem occurs.
To overcome this problem, various color separation techniques have been proposed. In color separation, a K signal is determined from the input Lab signals under prescribed conditions, and then, a set of CMYS signals calorimetrically consistent with the Lab signals are determined, using a mapping function modeling the relation between the CMYK outputs and the Lab signals.
For example, Japanese Patent No. 3360358 (Publication 1) discloses a technique for converting color value information to 4-dimensional CMYK values. In this publication, mapping conversion means is used to determine CMY output signals based on the K output level and the input color value information of the uniform color space. The K output level is varied in a stepwise manner, and a set of CMY output signals corresponding to the input color values is estimated every time the K output value is updated to determine the range (Kmax and Kmin) of the K output level. The maximum K value (Kmax) is multiplied by a coefficient to determine a definitive K output value so as not to be below the minimum level of black (Kmin). The final CMY output values are determined using the K output value, by use of the mapping conversion means. Since the mapping conversion is calorimetric conversion, the input color values are faithfully reflected in the CMYK signals.
Another publication JP 5-292306A (Publication 2) discloses a method for determining an actual ink amount. This method is similar to the above-described method, and a black adjusting coefficient is determined according to the pixel characteristics of the input image, and an actual amount of black is determined based on the maximum possible black amount and the black adjusting coefficient.
Still another publication JP 10-173948A (Publication 3) discloses a technique for realizing high-resolution color reproduction by further optimizing the color separation in accordance with the input image characteristics. In this publication, a gray histogram of the image is created to acquire analytical data including color distribution, the brightest point and the darkest point. A black version (k version) control amount is set with respect to the analytical data such that the printing stability becomes satisfactory. The k version control amount reflects the condition of k version constraint to perform color separation to CMYK values.
Meanwhile, image reproducing apparatuses using a light black (Lk) ink with a lower density than an ordinary black ink have been developed. See, for example, JP 2003-192967A (Publication 4). Such image reproducing apparatuses are advantageous in the viewpoints of:                (1) reducing metamerism under the environment using various types of light sources; (2) reducing granularity; and (3) expanding the color reproducible range.        
If the method of JP Patent No. 3360358 for determining an output signal level so as to be calorimetrically consistent with the input color value is applied to an image reproducing apparatus using a light black ink, color separation becomes difficult because there are still many combinations of CMY and light black (Lk) equivalent to the input color value existing even if a black output level is determined.
A conventional color separation for light black (Lk) ink is disclosed in, for example, JP 2002-67355A (Publication 5). The input image is converted to a set of CMYK signals consistent with the coloring characteristics of the output device, after having been subjected to Log transform, k generation and masking. The CMYK signals are further subjected to gamma conversion and multi-level error spreading for quantization, and the quantized data are transformed at each of the quantization levels using pallet matrix (referred to as pallet conversion). After the pallet conversion, each pixel level is converted to three-level signals representing no ink, light ink, and dark ink. In this manner, color separation into dark and light inks is performed through pallet conversion.
Another publication JP 2001-277552A (Publication 6) proposes a technique for performing color separation using light black ink for those colors that are likely to become granular if using black ink with an ordinary brightness.
The color separation disclosed in Publication 4 (JP 2003-192967A) cannot make use of the advantages of light black. This problem is explained below.
In a color image reproducing apparatus using ordinary four colors of CMYK, granularity is generally degraded because dots become noticeable when black ink is used with respect to a highlight color image, as compared with using chromatic colors. For this reason, black ink is not used for highlight colors.
Such problems in image quality can be reduced when using light black ink because dots are less noticeable than ordinary black ink. Accordingly, color separation may be performed at a high black ratio with an increased amount of black components, by appropriately combining black and light black. If such a high black ratio color separation is realized, various effects of improving the image quality can be expected.    (1) Metamerism is reduced. Since the reflectance spectrum characteristic of black ink is flat, the color becomes stable, even if the observation light source varies, by reproducing a gray color using black and light black.    (2) Stability with respect to change in engine is improved. If a gray color is reproduced using only black and light black, the gray color is not colored even if the engine changes, and therefore, color reproduction is performed with satisfactory color balance.    (3) Ink consumption is reduced. A color reproduced by three colors (CMY) in the conventional technique is replaced by light black, accordingly, the overall ink consumption can be reduced.
In Publication 4, color is separated into light black and black at the palette converting unit, so that both ink consumption and the image quality cannot be satisfied simultaneously. This problem is explained with reference to FIG. 4. FIG. 4 is a chart showing black output levels (indicated by the solid lines) and light black output levels (indicated by the dashed lines) with respect to the white-black axis and the blue-black axis.
First, consideration is made of five color reproduction along the white-black achromatic color axis. Along the achromatic color axis, if black is used for a highlight color, granularity becomes conspicuous because of a large difference in brightness from the white background. Accordingly, in order to perform color reproduction at a high black ratio, color reproduction is performed using only light black ink on the highlight side, and black ink is jetted when the density has increased to some extent.
Next, along the blue-black axis, the density of a blue color reproduced by a combination of magenta and cyan is high. Granularity is inconspicuous even if black is applied over the blue color because the microscopic difference in brightness is small and dots are not perceptible. Accordingly, black ink can be used from the beginning.
On the other hand, from the viewpoint of ink consumption, using black, rather than light black, is advantageous. Especially in printers, the total ink consumption may exceed the upper limit when using too much light black due to constraints on the printers concerning the total amount of ink to be ejected, and consequently, the color reproducible range is narrowed. For this reason, the color separation illustrated in FIG. 4, in which black ink is used from the beginning for the blue-black line and light black is used only for reproduction of a color near black, is required. Thus, using black ink and light black ink, the color separation scheme has to be optimized in response to the input color values.
However, the color separation using palette conversion is adapted to conduct color separation into a light black component and a black component, based on a k-signal generated after multilevel error spreading. For this reason, it becomes impossible to adjust the color separation levels of black and light black according to the input color value to be reproduced. Thus, the image quality and ink consumption requirement cannot be achieved simultaneously.
Still another publication JP 2001-277552A (Publication 6) discloses an apparatus that determines light black and black taking granularity into account; however, there is no clear description about creation of a color conversion table. Inferring implementation of this technique, a pattern is determined, taking granularity and ink duty into account, for three cases, that is, CMY color reproduction, CMYLk color reproduction, and CMYK color reproduction. Under the use of light black ink for a highlight color, it is necessary to replace light black ink by black ink when the brightness decreases to a certain extent. If light black is switched suddenly to black at the threshold level of brightness, pseudo outlines appear and the image quality is degraded, and therefore, the black level and the light black level have to change continuously, as explained above with reference to FIG. 4. However, Publication 6 does not describe continuously regulated color separation at all.